1. Field of the Invention
This invention relates to a machine tool, more particularly to a spindle head of a machine tool, such as a CNC machine tool, which permits adjustment of the preload applied to an anti-friction bearing so as to suit the rotation speed of a tool spindle.
2. Description of the Related Art
A conventional spindle head of a machine tool includes a tool spindle mounted rotatably in a housing. A tool member is connected to a front end of the tool spindle by a four-jaw chuck. An actuating rod is mounted in the tool spindle to actuate the chuck to clamp or release the tool member when the actuating rod is moved axially relative to the tool spindle. A spindle bearing assembly is interposed between the tool spindle and the housing, and includes front, intermediate, and rear anti-friction bearings for facilitating the rotation of the tool spindle relative to the housing. In this construction, the preload applied on the bearings is constant, thereby resulting in insufficient rigidity when the tool spindle is running at a lower rotation speed.
To solve this problem, another conventional spindle head of a machine tool has been developed to include a preload actuating member which includes a force transmitting member mounted between the housing and the front anti-friction bearing, and front and rear inlets disposed forwardly and rearwardly of the actuating member. As such, when a lower speed of the tool spindle is intended, hydraulic liquid can be introduced from the rear inlet so as to move the force transmitting member forwardly to force the front anti-friction bearing axially, thereby increasing the rigidity of the front anti-friction bearing. When the tool spindle is running at a higher rotation speed, another hydraulic liquid can be introduced from the front inlet so as to move the force transmitting member backwards to reduce the preload applied on the front anti-friction bearing. Therefore, by means of the movement of the force transmitting member relative to the front anti-friction bearing, the preload on the bearing can be adjusted. Moreover, the amounts of the inlets can be increased to adjust the preload to different levels.
However, since the diameters of the bearings are the same, the circumferential speed of the spindle bearing assembly, that is related to the pitch diameter of the bearing and the rotating speed of the tool spindle, is increased when the tool spindle is running at a higher rotation speed, thereby resulting in overheating of the bearing and eventual reduction in the service life of the same.
The object of the present invention is to provide a machine tool in which two anti-friction bearings of different diameters are selectable for use on a tool spindle so as to suit for the rotation speed of the tool spindle.
According to this invention, the machine tool includes a housing which has an annular inner wall extending in an axial direction. The annular inner wall includes a front annular inner wall segment, an intermediate annular inner wall segment which has a diameter smaller than that of the front annular inner wall segment, and an annular shoulder portion which is interposed between the front and intermediate annular inner wall segments and which extends in a radial direction relative to the axial direction. A tool spindle is mounted in and is rotatable relative to the annular inner wall, and includes a front spindle segment which is radially spaced apart from the front annular inner wall segment, an intermediate spindle segment which is radially spaced apart from the intermediate annular inner wall segment and which has a diameter smaller than that of the front spindle segment, and a rear spindle segment at the opposite side of the intermediate spindle segment relative to the front spindle segment. A tool member has a clamped portion which is disposed in the front spindle segment and which is releasably clamped by a clamping member, and a free tool portion which extends from the clamped portion axially and outwardly of the front spindle segment. An actuating member is mounted in and is axially movable relative to the tool spindle, and includes a front actuating end which is disposed in the front spindle segment to actuate the clamping member to clamp or release the clamped portion of the tool member when the actuating member is moved axially relative to the tool spindle, and a rear actuated end which extends from the front actuating end in the axial direction and rearwardly and outwardly of the rear spindle segment so as to permit external operation of the axial movement of the actuating member. A first anti-friction bearing is interposed between the intermediate annular inner wall segment and the intermediate spindle segment, and has a first outer diameter sufficient to facilitate rotation of the intermediate spindle segment relative to the intermediate annular inner wall segment. A second anti-friction bearing is interposed between the front annular inner wall segment and the front spindle segment, and has a second outer diameter larger than the first outer diameter of the first anti-friction bearing and sufficient to facilitate rotation of the front spindle segment relative to the front annular inner wall segment. A preload actuating member is disposed to force the second anti-friction bearing in the axial direction to take up the slack in a non-actuated state of the second anti-friction bearing when the tool spindle is running at a higherspeed, thereby resuming the anti-friction function of the second anti-friction bearing when running of the tool spindle at a lower speed is intended.